Chemical detection is often mandatory for industrial or safety applications and simple, reliable sensors should be implemented for process control or for security monitoring.
A number of chemical sensors for detection of chemicals are already known in the art, based on changes in characteristics such as physical, chemical, electrochemical or optical properties.
Chemical detection may be performed using electronic methods. For example, composite polymers having their electrical impedance changing upon exposure to vapors (e.g. commercial products made by the company Cyrano Sciences Inc.) may be used for this purpose. U.S. Pat. No. 5,512,882 (Stetter et al.), U.S. Pat. No. 4,631,952 (Donaghey), and U.S. Pat. No. 5,238,729 (Debe) show examples of chemical sensors of this type. In general, these types of sensing methods require a large variety of polymers or other types of materials with a selection of responses depending on the chemical species to detect, making them more complicated to produce and to use.
U.S. Patent No. 4,846,548 (Klainer), U.S. Pat. No. 5,828,798 (Hopenfeld), U.S. Pat. No. 6,278,106 (Muto et al.), U.S. Pat. No. 4,834,496 (Blyler, Jr. et al.), U.S. Pat. No. 5,436,167 (Robillard), U.S. Pat. No. 4,699,511 (Seaver), U.S. Pat. No. 4,940,328 (Hartman), U.S. Pat. No. 6,007,904 (Schwotzer et al.), U.S. Pat. No. 5,783,836 (Liu et al.), U.S. Pat. No. 5,015,843 (Seitz et al.), U.S. Pat. No. 5,308,771 (Zhou et al.), U.S. Pat. No. 4,998,017 and Re. 35,355 (Ryan et al.), U.S. Pat. No. 5,525,800 (Sanghera et al.), U.S. Pat. No. 4,732,480 (Fortunato et al.) and European patent EP 0 536 656 (Guenter et al.) show examples of optically based chemicals sensors and apparatus including fiber optic chemical sensors (FOCS).
A number of these FOCS use changes in the guiding properties of the optical fiber, including transmission parameters such as intensity, ellipticity and reflective or refractive angles. Many of the optical methods involved in the above sensors and apparatus require specific cladding or coating materials depending on the chemical species to be detected, which make them not very practical in industrial applications where different chemical species may be present. Some difficulties may arise during development of such chemically reacting cladding or coating such as compatibility of the reactive molecules with the desired refractive index range value, or adhesion problems between the core and the reactive cladding or coating of such fibers. Their applications may thus be limited to specific configurations.
Many of the optical chemical sensors use a spectroscopic approach and rely on light absorption at specific wavelengths to detect chemical species. Such spectroscopic approaches can be a very powerful tool for chemical characterization and quantification but are usually expensive and difficult to implement, and require usually some good knowledge for adjustments and for data interpretation.
In order to increase the contact surface of the sensor with chemicals to be detected, porous materials with high surface area are often used. Capillary condensation and use of porous silicon as sensing material are described in the literature (see e.g. Gelb, L. D. et al., “Phase separation in confined systems”, Rep. Prog. Phys. 62, 1999, pp. 1573-1660; Gross, E. et al., “Highly sensitive recognition element based on birefringent porous silicon layers”, J. Appl. Phys. 90 No. 7, 2001, pp. 3529-3532; Liu, R. et al., “Novel porous silicon vapour sensor based on polarization interferometry” Sensors and Actuators B 87, 2002, pp. 58-62; Gao, J. et al., “Vapor sensors based on optical interferometry from oxidized microporous silicon films” Langmuir 18, 2002, pp. 2229-2233; Gao, J. et al., “Porous-silicon vapour sensor based on laser interferometry” Appl. Phys. Lett. Vol. 77 no6, 2000, pp. 901-903; Canham, L. T., “Properties of Porous Silicon”, Canham L. Ed., EMIS Data reviews series No. 18, 1997, INSPEC publ., pp. 154-157; Bjorklund, R. B. et al., “Color changes in thin porous silicon films caused by vapor exposure”, Appl. Phys. Lett. 69 (20), 1996, pp. 3001-3003; Zangooie, S. et al., “Vapor sensitivity of thin porous silicon layers”, Sensors and Actuators B 43, 1997, pp. 168-174; Zangooie, S. et al. “Reversible and irreversible control of optical properties of porous silicon superlattices by thermal oxidation, vapour adsorption, and liquid penetration” J. Vac. Sci. Technol. A 16(5), 1998, pp. 2901-2912); as well as in the U.S. Pat. No. 6,130,748 (Kruger et al.), U.S. Pat. No. 6,248,539 (Ghadiri et al.), U.S. Pat. No. 5,338,415 (Sailor et al.) and U.S. Pat. No. 5,453,624 (Sailor et al.). Porous glass can also be used as described in U.S. Pat. No. 5,250,095 (Sigel, Jr. et al.) and U.S. Pat. No. 6,375,725 (Bernard et al.).
In cases where porous materials such as porous silicon films are used, fragility due to high porosity (e.g. usually over 50-80%) associated with small film thicknesses (e.g. typically 10-100 μm) makes them brittle and less attractive for industrial applications where robust sensors are required, especially if they must be embedded inside an absorbent material. Besides aging problems related to surface oxidation and chemical stability, another drawback of porous silicon sensors is that spectral shifts occur in the far red and near infrared region (˜800-1700 nm) which means that the human eye could not be used as a light detector. However in some cases, color changes, characterized by ellipsometry, are related to the refractive index of the solvents condensing into the pores and replacing air. Since lower partial pressures of solvent cause no color changes in the film, only the variation in the ellipsometric angles at certain energies could be applied to sensing applications.
Air purifying devices, including air purifying respirator cartridges and canisters, are widely used in the civil and military industries to protect the workers against harmful effects of toxic materials. Such devices usually consist of a filter chamber filled with adsorbent material that traps (e.g. adsorbs or absorbs) vapors or gases on its surface or within its porous structure. As the adsorbent material is completely filled, the air-purifying device loses protective capability for the user against the contaminant. This could have dramatic effects, especially when the contaminant has poor warning properties, e.g. if its odor, taste or irritation limit is greater than the permissible exposure limit or if there is insufficient toxicological data to determine an exposure limit.
In establishing new certification standards in 1984, the U.S. National Institute for Occupational Safety and Health (NIOSH) encouraged the development of active end-of-service-life indicators. Such indicators should detect the presence of contaminants and provide an unambiguous signal warning the user that the filter of the air-purifying device is almost exhausted. Examples of chemical sensors proposed for use as end-of-service-life indicators are shown in U.S. Pat. No. 4,154,586 (Jones et al.) and U.S. Pat. No. 4,530,706 (Jones), U.S. Pat. No. 4,684,380 (Leichnitz), U.S. Pat. No. 4,326,514 (Eian), U.S. Pat. No. 5,659,296 (Debe et al.), U.S. Pat. No. 4,155,358 (McAllister et al.), U.S. Pat. No. 4,146,887 (Magnante), U.S. Pat. No. 4,847,594 (Stetter), U.S. Pat. No. 6,375,725 (Bernard et al.) and in international application No. WO 02/22237 (Curado et al.).
End-of-service-life indicators may involve a visual color change that warns the user to replace the filter. Such color changes are sometimes induced by chemical reactions of a usually single use color indicator. One drawback of such chemical color indicators is that they are usually very specific to the chemical or class of chemicals (such as acids) they should react with.